Cathode-ray tubes of the focus-mask variety



Oct. 24, 1961 R. H. GODFREY CATHODE-RAY TUBES OF THE FOCUS-MASK VARIETY Filed Jan. 27, 1958 INVENTOR. RIEHARD H. EEIDI-REY B 72% WW 13,005,927 CATHODE-RAY TUBES OF THE FOCUS-MASK VARIE Richard H. Godfrey, Landisville, Pa., assignor to Radio Corporation of America, a corporation of Delaware Filed Jan. 27, 1958, Ser. No. 711,249

5 Claims. "(CL 315-9) cathode-ray tubes are herein referred to as being of the focus-mask variety.

If the screen and the envelope watis of a tocuemask fbetweenthe planeef-defieetieneed themask ef the tube tube are operated at the same high potential and the mask is operated at a lower potential, the field between the tubes plane-of-deflection and the mask decelcrates the beam-electrons, and the field between the mask and the screen accelerates the beam-electrons. Consequently,

1 Tc Patented oc 24, 1961 screen along curved paths that terminate at the screen-end of straight lines drawn from the screen through the centers of the mask apertures substantially to points whereat the axes of said electron-beans pass through said planeof-deflection. With the principal amount of misregister thus removed, the light-optical methods of the Epstein, Kaus, and Van Ormer disclosures can easily be applied if necessary to obtain a higher degree of correction.

The invention is described in greater detail in con- 10 nection with the accompanying single sheet of drawings,

wherein:

FIG. 1 is a plan view, partly in section, of a 3-gun tri-color kinescope of the focus-mask dot-screen variety which is provided, in accordance with the invention, with a plurality of field electrodes circumscribing the space and an appropriate energizing circuit for said field electrodes; the drawing being marked with equipotential lines indicative of the decelerating electrostatic field in said 0 space and with other lines illustrative of the path of the the electrons before reaching the mask are refracted away from the tube-axis and, after passing through the maskapertures are refracted toward the tube-axis. This refraction phenomena may be considered to be separate from the focusing effect produced by the mask-apertures acting as lenses. Refraction of the beam-electrons away from the tube-axis as they approach the mask is beneficial in that it reduces the scanning power required of the yoke. However, unless the outward refraction eflect before the mask is exactly off-set by the refraction in the other direction, between the mask and screen, a phosphor dot (or line) screen prepared by optical projection in a lighthouse (as used in the manufacture of mosaic-screens for use in shadow-mask tubes) cannot be employedsince the terminal points of the light paths and the electron paths on the phosphor-screen will not coincide. If, for example, the mask of a 90 shadow-mask tube is operated at a low potential, as required in the operation of a focus-mask tube, the optical and electron terminal points for rays passing through a given mask-aperture near the edge of the tube are out of register by about 0.200. It

might at first glance appear that this amount of misregister could be corrected by doing two things: (1) Reducing the mask to light-source distance during the screenplotting operation to correct the linear component of the misregister and (2) interposing an optical lens between the light source and mask (as taught by Epstein, Kaus an Van Orrner in US. Patent 2,817,276, see alsocopending application Serial No. 585,254, now U .S. Patent No. 2,885,935) to correct the non-linear component. In trying to do these two things, however, it has been found 1) that the light source-to-mask distance is too short to be practical due to severe non-uniformity of illumination caused by inverse square law fall-off, increased magnification of light source, etc. and (2) that the optical consequently diflicult to make and to use.

The present invention. may be said to be predicated in part upon an appreciation of the fact that the solution field electrodes circumscr-ibing the space between the plane-of-deflection and the mask operate to modify or shape the decelerating electrostatic field in said space. Proper adjustment of these fields, and of the field in the mask-to-screen space, subjects the beam-electrons to refractive effects which cause said electrons to approach the-- beam-electrons through said decelerating field and,

FIG. 2 is an enlarged fragmentary view in perspective of the screen-unit of the tube of FIG. 1.

The 3-beam, tri-color, focus-mask, dot-screen, colorkinescope 1 shown in FIG. 1 comprises an evacuated allglass envelope having a bulbous cone portion 3 whichterminates at one end in a neck 5 and, at its opposite end,

in a front-end portion or cap 7. The cap 7 terminates in a curved face-plate 9, the concave inner surface of which comprises the mosaic screen 111 of a bi-part screenunit 11, 13 of the focus-mask variety.

The mosaic screen 11 (see FIG. 2) comprises a multiplicity (usually 300,000 or more) of triads (i.e. groupsof-three) of red (R), blue (B) and green (G) colorphosphor dots. The phosphor-dots are here arranged in 40 response characteristic.

a hexagonal pattern; that is to say each dot is surrounded by six other dots, alternate ones of said other dots being of a second color-response characteristic and the inter- ,mediate ones of said other dots exhibiting a third color- An electron-transparent, lightreflecting, metallic (e.g. aluminum) film 11 (FIG. 1) covers the entire target surface of this dot-like mosaicscreen. A suitable electrical terminal, exemplified by the lead-wire 11w extends from the screen electrode 11 to the exterior of the envelope where it is shown connected to a source of voltage, V.

The other element, or focus-mask, of the bi-part screen-unit 11, 13 preferably comprises a suitably curved thin-metal plate 13 containing a multiplicity of apertures 13a arranged in the same systematic (hexagonal) pattern as the phosphor screen-dots of a given color, there being one mask'aperture for each triad (RBG) of dots. The mask has an integral rim or frame 15 and is supported about said rim on three or more pegs 15p which 5 project radially inward from the inner surface of the cap 0 lens required would have to be exceedingly strong and portion 7 of the envelope. The connection between the pegs 15p and the frame 15 is such as to permit the mask to be removed from the cap during the three emulsioncoating anddeyeloping operations incideutto laying down i the three color-phosphors (RBG) on the glass screenplate 9. The mask -'13 is maintained at a potential considarebly lower than that of the screen-electrode 11 in order to establish a beam-focusing field (not shown) about each mask-aperture in the space q between the mask and the screen. The required voltage is here shown as applied to the mask 13, from the source V, through a lead 13w connected to the mask-frame 15. Appropriate voltages are marked on the drawing.

a The tubular glass neck 5 of the envelope lcontains a 0 battery of three electron-guns 17r, 17b, and 17g each of which is allotted to a particular screen-color. The guns are here shown arranged delta (A) fashion about the long 2,595,548) so that the three beams r, b, and g converge adjacent to the surface of the mask 13 where their paths cross and proceed to the different color-phosphor dots. (In order to simplify the drawing only the path of the red beam r is marked within the bulbous part 3 of the envelope.) Alternatively, the guns may be arranged in line, or a single beam may be employed, in which latter case auxiliary means may be provided as, for example, in Jenny 2,611,099, for sequentially shifting the single beam to positions corresponding to that of the several beams in a multigun tube.

As is now more or less standard practice the red, blue and green beams are subjected to dynamic convergence forces for maintaining them converged at'or near the surface of the mask throughout their scanning movement. In the instant case the dynamic convergence forces are applied to the separate beams by three internal pole pieces 19 actuated by small electro-magnets 21 in the manner described in greater detail by Albert M. Morrell in US. Patent 2,752,520. US. Patent-2,751,'5l9'to Albert M. Friend may be referred to for other electromagnetic (and electrostatic) types of dynamic beam-convergence means. In FIG. 1, as in the Schroeder patent (2,595,548) the required horizontal and vertical scanning movements are applied to all three electron-beams from the guns 17 by a common deflecting yoke 23. The normal plane-of-defiection is indicated in FIG. 1 by a straight line PP extending through the yoke in a direction normal to the central axis xx of the tube.

As thus far described the color-kinescope shown in the drawings is of more or less conventional construction. The indicated mask and screen voltages used to establish a focusing field about each mask aperture (13a FIG. 2) are also conventional. With the mask and the screen energized in the manner above described the beam-electrons pass through a decelerating electrostatic field in their transit from the electron-guns 17 to the mask 13 and an accelerating electrostatic (focusing) field in traversing the mask-to-screen space (q FIG. 1). If the entire electrode arrangement in this tube were in fact conventional, the second-anode (25 in the drawing) would extend (for example in the form of a conductive coating on the inner wall of the bulb) from the leading end of the gun(s) 17 to a region adjacent to the mask-frame and would ordinarily be operated at the same ultor potential as the screen. With such an arrangement the terminals of the equipotential surfaces (of which such a lens-field is comprised) would all be funneled into the space between the edge of the mask frame and the adjacent edge of the secondanode. The equipotential surfaces in such a conventional tube thus exhibit changes in contour which subject the beam-electrons to a high degree of outward refraction. The refraction becomes very high in the region where the beams approach the limits of their scanning movements. As previously pointed out, while it is possible to compensate for the resulting misregister by the use of a very strong optical-lens during the screen plotting operation it is not always practical to do so.

v The present invention provides 'a series of field electrodes (later described) circumscribing the space between the plane-of-defiec-tion and the mask. These added fieldelectrodes, when appropriately energized, operate to so alter the contour of the equipotential surfaces e (of which the decelerating electrostatic field in said space is comprised) that the eleotron beams are not subject to an excessive degree of refraction but on the contrary are refracted outwardly only a sufiicient amount along curved paths (r) to compensate for the inward refraction between mask '13 and screen 11 such that a straight line A- A drawn from the point of impact on the screen 11 through the center of the mask-apertures 13a projects back substantially to the appropriate color centers R, B, and G (FIG. 2) in the deflection plane P--P for all screen radii.

.substantially zero misregister.

In the instant case, one of the aforesaid series .of field electrodes comprises the metal frame or rim 15 upon which the mask is supported on the other electrodes in the series comprise three electrically conductive bands 25, 26, and 27 disposed on the inner surface of the bulb in axially spaced insulating relationship with respect to the mask frame 15. The first conductive band 25 may form part of the second-anode of the tube and extends from the guns 17 (to which it is connected), through the plane-ofdeflection PP, into the bulb 3 a limited distance (say 3.2 inches in a 21" tube as measured from said plane P--P along the central axis of the tube). This first field-electrode 25 is preferably maintained at the same (ultor) potential as the screen, as is indicated in the drawing by the position of its lead-wire 25w on the voltage source V. The second conductive band 26 is spaced, say .55 inch from the leading edge of the first band 25 and is about 1.7 inches wide as measured along the tube-axis X-X. The lead wire 26w for this second field electrode 26 is ordinarily connected to the same point on the voltage source V as the lead 13w for the mask 13 and its frame 15, though in some cases, e.g. where it is necessary or desirable to alter the position of the scanning yoke 23 to minimize over-scanning, the field-electrode 26 may be operated at a potential lower or higher than the mask, but nevertheless several thousand volts below the ultor potential applied to the screen. The third conductive band 27 is spaced approximately the same amount from the second band 26 as the second from the first (25) and extends up the bulb wall to a position whereat its leading edge lies approximately .200" from the plane of the mask frame 15. The lead wire 27w for this third band 27 is connected to the same ultor voltage point on the source V as the screen 11 and the first band 25.

The manner in which the gap field former of the present invention operates to straighten out the register curve will the more readily be understood by comparison with what takes place in the operation of tubes of the prior art employing a solid field former. (The term gap field former as applied to tubes of the present invention is suggested by the presence of a high voltage electrode or gap in what would otherwise be a continuous low voltage section from electrode 26 to the mask.) A solid type field former extends the low voltage on the mask toward the gun at the edge of the solid angle 0ccupied by the scanning beam. Thus the equipotential surfaces of the field are so bent that the beam, as it is deflected by the yoke through increasing screen radii, approaches said surfaces more nearly perpendicularly and, consequently, is refracted less outwardly. Although this results in an increasing amount of compensation for the mask-to-screen field, and a resultant reduction in misregister, nevertheless a point is reached where further extension of the field former into the cone over-compensates at the screen-edge and a non-linearity of remaining misregister vs. screen radii results. The gap type field former improves upon this action by straightening out the register curve and makes its possible to attain This is done by interposing a high voltage field (on electrode 27) between the low voltage extension (electrode 26) and the low voltage mask-assembly (1315). The effect of such a field distribution is to straighten the equipotential surfaces (FIG. 1) in the area immediately preceding the mask. The action on the beam is then as follows: (1) It is still given an initial outward refraction in its transit from yoke to mask. Hence the saving in deflection power is maintained. (2) The beam path is then straightened out by the field due to the gap structure so that the refraction of the beam between the gun and mask substantially compensates for the refraction between the mask and screen.

Focus-mask tubes constructed and operated in accordance with the present invention possess many advantages, addition to improved register, over the focus-mask tubes'of the prior art. By way of. example, tubes of'the present invention exhibit relatively low sensitivity to voltage fluctuations. This is so because the closer the auxiliary fields (provided by the electrodes '25, 26, 27, come to compensating for the retracting effect of the 5 field (between the mask and screen), the more independent said fields become to fluctuations in the voltages addition to so-called dynam'e degrceping twhich theta small electro-magnets 21 are designed to correct) there exists (in large deflection angle tubes) a type of a degrouping known as static degrouping. Static degrouping increases with increasing deflection angle even before the dynamic component is added, is periodic with azimuthal angle, and occurs with a definite phase relationship between the three beams. It can be shown that, to the first approximation: 25

Over the entire range of possible approach angles the slope +2 sin 20] is a positive one, hence the spacing between beam center and trio center, for equal convergence angles, decreases with a decrease in the component of trio approach angle which is co-planar with the convergence angle. Since the electrostatic fields provided by the auxiliary electrodes of the present invention operate to reduce this angle, static degrouping is reduced.

Yet another advantage of the present invention is that it lends itself readily to the use of tube-envelopes of various shapes. Factors influencing the shape of the tube envelope are strength, stability, and appearance (such as rectangular, etc.). Optimum shape as regards these factors may be something much less than optimum as far as the beam refraction is concerned since, in the absence of any other field forming device, the bulb walls alone will shape the electrostatic field and thus determine the final net beam refraction. Inserting the field-former electrodes within the envelope permits the designer to: (1) extend the low voltage field as far down as necessary to achieve refraction compensation for small radii and (2) control the Width and position of the high voltage gap to provide the proper refraction compensation for the remaining radii. These dimensions can be systematically varied by azimuthal position when refraction requirements so dictate. Greater flexibility in the design of tube shape for focus-mask tubes is therefore permitted. :5 For example, it has proven practical to change over from a 90 metal focus-mask tube to a 90 glass focus-mask tube with resultant large changes in bulb shape. The additional design variables, which the gap field former provides, ensures better register, degrouping, etc. 7

In addition to the factors mentioned previously, a gap field former when connected to a voltage supply independent of the mask frame assembly can provide another degree of freedom in focus-mask operation. Altering the voltage on the field former produces a beam register change not unlikethat provided by an axial movement of the yoke; hence, the yoke may be abutting the envelope funnel at all times and small register deviations from tube to tube can be compensated for by readjustment of the field former voltage for that particular tube. Furthermore raster pincushioning may be altered by changing the voltage on the gap field forming ring alone.

What is claimed is:

1. In a multi-beam cathode-ray tube of the focus-mask variety having a mosaic screen the individual elements of which are disposed at the terminals of substantially straight lines drawn through the mask-apertures to respectively different points in the tubes plane-of-deflection and wherein said electron beams normally traverse doubly curved paths in their transit to respective ones of said individual screen elements by reason of (I) the refi active efiectfofm deceleratingelectmstatic fieldin the space between said plane-of-deflection and said mask and (II) the unequal opposite refractive effect of "an accelerating electrostatic focusing field in the mask-toscreen space, the improvement which comprises: electrode means including at least three spaced apart field electrodes circumscribin-g said first-mentioned space, and means for applying voltages to said field electrodes to establish a potential distribution among said field electrodes such that said unequal opposite refractive eifects of said accelerating and decelerating fields upon said electron-beams are rendered substantially equal, whereby the terminals of said doubly curved beam paths are caused to coincide at said screen substantially with the terminals of said straight lines.

2. The invention as set forth in claim 1 and wherein said electrode means is constituted in part by a means supporting the mask out of contact with the inner surface of said tube and, in part, by plurality of electrically conductive bands disposed adjacent to said inner surface in axially spaced relationship with respect to each other and with respect to said support means.

3. The invention as set forth in claim 1 and wherein alternate ones of said field electrodes are adapted to have a common operating potential applied thereto and intermediate ones of said electrodes are adapted to have a different common operating potential applied thereto.

4. The invention as set forth in claim 3 and wherein the screen-electrode of said tube is adapted to have an ultor potential corresponding to the first mentioned of said common operating potentials applied thereto.

5. A cathode ray tube having a plane-of-deflection comprising a screen-electrode having a mosaic tar-get surface, an electrode mounted adjacent to said mosaic surface and containing a pattern of apertures which is systemmatically related to the pattern of elemental areas of which said mosaic is comprised, electron-gun means mounted adjacent to said plane-of-deflection in a position to scan said screen-electrode through the apertures in said apertured electrode, a plurality of ring-like alternate and intermediate electrodes disposed in axially spaced relationship in the space between said plane-ofdeflection and said apertured electrode and through which electrons pass in their transit to said apertured electrode, means for applying a common ultor potential to said screen-electrode and to said alternate ones of said ringlike electrode, and means for applying a common lower operating potential to said apertured electrode and to the intermediate ones of said ring-like electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 2,131,563 Knoll Sept. 27, 1938 2,345,282 Morton Mar. 28, 1944 2,548,118 Morton Apr. 10, 1951 2,580,250 Smith Dec. 25, 1951 2,590,764 Forgue Mar. 25, 1952 (Other references on following page) 8 Francken Nov. 12, 1957 Du Four June 3, 1958 FOREIGN PATENTS Switzerland July 15, 1937 Great Britain May 24, 1943 Sweden Dec. 7, 1943 

